1. Field of the Invention
The present invention relates to a controller for an optical scanner including an angle detecting unit and adapted to position a mirror supported by an axis of rotation.
In an optical scanner used in the laser marking and the laser drilling through a printed wiring board using a laser beam, a mirror mounted on the rotation shaft is rotated by an electric motor having the mirror therewith to change the angle of the mirror, whereby a laser beam emitted from a laser oscillator is irradiated on a predetermined position of an object to be fabricated.
2. Description of the Related Art
FIG. 12 is a schematic view showing the construction of a movable part in the optical scanner. A mirror 11 is mounted on an end of a rotation shaft 12. The rotation shaft 12 is supported by a bearing 14 and a bearing 15 and is rotated by suffering the driving torque by a moving coil 13 incorporated integrally with the rotation shaft 12 to be positioned to a predetermined angle. Hereinafter, the mirror 11, the rotation shaft 12 and the moving coil 13 which are rotated integrally are collectively referred to as an optical scanner 1xe2x80x3 for short when applicable.
The optical scanner 1 has an angle detecting sensor (not shown) for detecting the rotation angle of the rotation shaft 12, e,g., a variable capacitance type sensor. The variable capacitance type sensor is configured in such a way that a dielectric plate mounted on a rotation shaft is rotated together with the rotation shaft between a set of two fixed electrode plates. Then, the angle of the rotation shaft is detected in the form of an electrical signal as the change in the electrostatic capacity between the electrode plates. The technique of such a variable capacitance type sensor, for example, is disclosed in U.S. Pat. No. 3,517,282, U.S. Pat. No. 4,864,295 and JP-A-7-55500.
In addition, JP-A-4-127981 discloses a technique that a laser beam for measuring an angle is irradiated on a mirror and the reflected light beam is detected by a linear sensor to control the mirror angle in the feedback control. Furthermore, JP-A-63-147138 discloses a technique that a reflecting surface is formed on a component (mirror mount) to be fixed a mirror on a rotation shaft and the light beam emitted from a light emitting device reflected from the reflecting surface is detected by a light receiving device, thereby positioning the mirror.
FIG. 13 is a servo block diagram showing a configuration of the optical scanner. In order that an angle detection signal 22 is negatively fed back to an angle desired value signal 21 supplied from a host controller to make zero a value of a tracking error signal 23, the tracking error signal 23 is integrated by an integral compensation circuit 24. In addition, in order to maintain the stability of this servo mechanism, the angle detection signal 22 is inputted to both a proportional compensation circuit 25 and a derivative compensation circuit 26, and the sum of the output signals from these circuits is subtracted from an output signal of the integral compensation circuit 24 to make the resultant signal a control input signal 27. A motor driving circuit 28 supplies a motor driving current 29 proportional to the control input signal 27 to the optical scanner 1. The motor driving current 29 flows through a moving coil 13, thus, the driving torque proportional to the current value acts on the moving coil.
In the case where holes are formed through a printed wiring board by the laser beam, in order to form the holes through the fine circuit pattern with high accuracy, it is necessary to make the positioning error which should be equal to or smaller than about 10 xcexcm when forming the holes. In addition, in order to shorten the laser drilling time, the enhancement of the drilling speed is required for the movement up to the drilling position of a next hole after completion of the one hole. For example, in the case where 1,000 holes are formed per second, the time required for the movement between the holes needs to be made shorter than 1 ms in average.
Now, the laser beam has the energy distribution and is made incident to the mirror 11 with the area extend held. For this reason, in order to form the high quality holes, it is desirable that the size of mirror 11 is large (i.e., the area thereof is large).
However, if the size of mirror 11 increases, then the torsional vibration and the flexural vibration of the rotation shaft 12 are both increased, which results in the impediment of the high response of the mirror positioning.
First of all, the description will be given with respect to the influence of the torsional vibration. If the size of mirror 11 increases, the moment of inertia around the rotation shaft 12 also increases, therefore, the natural frequency of the torsional vibration acting by the rotation shaft 12 decreases. The natural frequency of the first mode is minimum. One node of the torsion is then present in the longitudinal direction of the rotation shaft 12 and the both sides sandwiching the node are subjected to the angular displacement in an opposite phase. In addition, the natural frequency of the second mode is low next to that of the first mode. Two nodes are therefore present in the longitudinal direction of the rotation shaft 12, and the both sides of the central part sandwiched between the two nodes are vibrated in the opposite phase with respect to that central part.
For example, when the angle detecting sensor is close to the mirror, the node of the torsional vibration is located between the angle detecting sensor and the moving coil 13, whereby the angle detecting sensor and the moving coil 13 may be subjected to the angular displacement in the opposite phase in some cases. In such cases, in the above-mentioned servo mechanism, the torsional vibration frequency component of the angle detection signal 22 is positively fed back, so that the control becomes unstable. Though the control bandwidth is desirably wider in terms of the characteristics of the high response of the mirror positioning and the suppression of the low frequency disturbance, the control bandwidth is limited due to the torsional vibration.
In addition, when the node of torsional vibration overlaps the position of sensor or is located close to the sensor, since the vibration mode thereof can not be observed by the angle detecting sensor, the stabilization can not be obtained in terms of the control and hence the accuracy of positioning the laser beam is reduced.
Next, the description will hereinbelow be given with respect to the influence of the flexural vibration. It is desirable that the movable part of the optical scanner shown in FIG. 12 is balanced around the rotation shaft 12. However, if the mass of mirror with respect to the two sides of the moving coil 13 in the longitudinal direction which is subjected to the driving torque or the mass of mirror with respect to the axis line of rotation shaft 12 is different between the left and right sides thereof, the mass difference becomes the unbalance weight. The flexural vibration is then generated on the rotation shaft 12 with the rotation shaft 12 supported by the bearing 14 and the bearing 15 due to the force of inertia caused by the unbalance weight resulting from the operation of the optical scanner 1. As a result, the mirror 11 vibrates in the parallel direction or in the perpendicular direction with respect to the mirror surface. In general, the optical scanner 1 does not have a sensor for detecting the flexural vibration of the rotation shaft 12 and an actuator for applying the force in the direction of the flexural vibration to the rotation shaft 12. In addition, in the feedback control made by the above-mentioned servo mechanism, it is impossible to attenuate the flexural vibration which has been generated once. For this reason, it is impossible to enhance the accuracy of positioning the laser beam.
In the light of the foregoing, it is therefore an object of the present invention to provide a controller for an optical scanner capable of reducing the torsional vibration and the flexural vibration to be generated on a rotation shaft having a mirror mounted thereon to shorten the time required to position the mirror and to enhance further the accuracy of positioning a laser beam.
In order to attain the above-mentioned object, according to the first aspect of the present invention, there is provided a controller for an optical scanner for positioning a mirror supported by a rotation shaft around the axis on the basis of an angle desired value and an angle detection value, wherein a correction value with respect to the torsional vibration of the rotation shaft is added to an integration value of the tracking error between the angle desired value and the angle detection value, reducing the torsional vibration of the rotation shaft.
In this case, the above-mentioned correction value can be made an output value of the transfer function up to the r-th order (but, r is positive integer) angular velocity of the torsional vibration of the above-mentioned rotation shaft due to the driving torque added to the above-mentioned rotation shaft, and furthermore, the output value of the above-mentioned transfer function can be calculated from the current value supplied to the motor by which the driving torque is generated.
In addition, there is provided a controller for an optical scanner for positioning a mirror supported by a rotation shaft on the basis of an angle desired value and an angle detection value, wherein a target trajectory is made a time function of a position, and also a specific frequency component is removed from the target trajectory, and from the sum of a target velocity and a target acceleration based on the target trajectory to make the resultant value the above-mentioned angle desired value, reducing the flexural vibration of the rotation shaft.
In this case, the above-mentioned specific frequency component can be decided as the natural frequency component of the flexural vibration of the rotation shaft, or as the natural frequency component of the torsional vibration of the rotation shaft.
According to the second aspect of the present invention, there is provided a controller for an optical scanner for determining an angle of a mirror supported by a rotation shaft on the basis of an angle desired value and an angle detection value, wherein the controller includes: a mirror angle detecting unit for detecting an angle of the mirror; a rotation shaft angle detecting unit for detecting an angle of the rotation shaft; a current detecting unit for detecting a current supplied to a motor for rotating the rotation shaft; and a torsional vibration stabilizing compensation unit for stabilizing the torsional vibration on the basis of the current value, wherein the value from the proportional compensation and the derivative compensation both using the rotation shaft angle detected value and the value from the torsional vibration stabilizing compensation unit are negatively fed back to the value integrally compensated the tracking error between the angle desired value and the mirror angle detection value to determine the current value supplied to the motor, reducing the torsional vibration of the rotation shaft.